1. Field of the Invention
This invention relates to a top coat material and the use thereof in lithography processes. More particularly, this invention is directed to a top coat material which is easy to apply, insoluble in water but soluble in developer, therefore can be removed in the develop stage. This top coat may be especially useful for immersion lithography in which a liquid such as water is used as the exposure medium between the lens fixture of an exposure tool and the photoresist-coated wafer.
2. Description of the Related Art
Traditionally, top coat materials have been used in photolithography as anti-reflective films on the top of a photoresist. The top anti-reflective coat (TARC) materials can prevent the multiple interference of light that takes place within the photoresist layer during exposure. As a result, the critical dimension (CD) variation of the geometrical features of a photoresist pattern that is caused by the variation in the thickness of the photoresist film can be minimized.
To fully take advantage of the anti-reflective effect of the top coat, the refractive index of the top coat material (nt) should be at about the square root of the multiplication of the refractive index of the exposure medium (nm) and the refractive index of the underlying photoresist (nr). If the exposure medium is air, as in the case for “dry” lithography, the optimal refractive index of the top coat material (nt) should be at about the square root of the refractive index of the underlying photoresist (nr) because the refractive index of air is roughly 1.
For ease of processing, classic TARC materials are designed to be soluble in both water and aqueous base developer so that they can be applied directly from water solution and subsequently removed by the aqueous base developer during the develop stage.
Numerous top coat materials have been developed to meet these two requirements of optimal refractive index and solubility. For example, U.S. Pat. Nos. 5,744,537 and 6,057,080 disclose aqueous-soluble TARC materials comprising a polymeric binder and a fluorocarbon compound, and which have nearly ideal refractive indexes on the order of 1.3-1.4. U.S. Pat. No. 5,879,853 also disclose a TARC material that is removable by a wet process. U.S. Pat. No. 5,595,861 similarly discloses a TARC comprising partially fluorinated compounds, which can also be water soluble. U.S. Pat. No. 6,274,295 discloses a TARC material comprising a light absorbing compound having a wavelength of maximum absorption higher than an exposure wavelength used to expose the photoresist. This TARC can also be water-soluble. Finally, U.S. Pat. No. 5,240,812 discloses a protective material for use as an overcoat film for acid catalyzed resist composition to prevent contamination from vapors of organic and inorganic bases. While not specifically disclosed as being a TARC, the overcoat can also be water soluble.
Immersion lithography offers the potential to extend the use of optical lithography to print smaller features. In immersion lithography, air is replaced by a liquid medium such as water between the lens and the wafer. Use of a medium with an index of refraction higher than air results in a greater numerical aperature (NA), and therefore allows printing of smaller features. See “Technology Backgrounder: Immersion Lithography,” published by ICKnowledge.com at http://www.icknowledge.com, May 28, 2003. See also L. Geppert, “Chip Making's Wet New World,” IEEE Spectrum, Vol. 41, Issue 5, May 2004, pp. 29-33.
For liquid immersion lithography, a top coat material can be used in between the exposure medium and the resist-coated wafer to prevent photoresist components from leaching into the immersion medium. A top coat material can also prevent the permeation of the exposure medium into the photoresist film. One of the requirements for the top coat material, of course, is its insolubility in the exposure medium. Preferably, the top coat material can also act as a TARC layer.
Water has been proposed as the exposure medium for 193 nm immersion lithography. Therefore, classic water-soluble TARC materials such as those described above can not be used as top coats for 193 nm immersion lithography. Moreover, since water has higher refractive index (1.47 at 193 nm) than air (˜1 at 193 nm), the optimal refractive index for TARC materials used for 193 nm immersion lithography is also higher than that of classic TARCs.
Thus, there remains a need for a top coat material that is insoluble in water but soluble in aqueous base developer, and also has desired optical properties so that it can also be used as a TARC.